Tinker Gnome: MAPP Gas Pop Gun

I share with you now my latest Tinker Gnome Project: The MAPP Gas Pop Gun. Like most of my tinker projects, it’s impractical, voids every warranty within a hundred miles, and has just as much chance of hurting me as of providing entertainment.

If you couldn’t care less about the construction details or my thoughts, just scroll down to the very end for pics and video of the pop gun in action.

Let’s get the obligatory “please don’t sue me” warning out of the way:

This post is for education and entertainment purposes only. DO NOT TRY THIS AT HOME! None of these items are being used for their intended purpose. MAP Pro gas is extremely dangerous stuff, and fire is an unforgiving beast. If something goes wrong, this thing could injure or even kill you. Even if everything goes right, this pop gun could still get you arrested if you use it in the wrong location. Ignore my warnings at your own risk.

The first concern I had with the original designs were the bulky gas cylinder hanging below the trigger. It’s both ugly and awkward. The torch model that everyone seems to be using (Bernzomatic TS4000 or TS8000) doesn’t have separate flow and ignition controls, so you have to press the trigger enough to let gas flow but not enough to actually set off the ignition spark. Not only does this make premature ignition a concern [insert immature joke here], but you also have no control over the flow rate of the gas. Yet another awkward design flaw in my opinion. The best solution I could find to all of these issues was the Bernzomatic TS99T, which can swivel around 360 degrees at the regulator. Turned so the flame is pointing backwards and horizontal, this allows the ignition switch to actually be pulled more like a real trigger. Without the vinyl tubing to carry the flame, this placement would cook my arm, so I had to make sure the tubing was well-secured. The arrangement also turns the cylinder nearly horizontal, providing me with less obtrusive mounting options.

My second concern was the use of plastic bottles in the design. They may be lightweight, cheap, and easy to come by, but they also warp and give off toxic fumes when exposed to flame. While the use of MAPP gas also requires good ventilation, I’d like to keep my lung damage to a minimum. Glass would be better in many respects, but it’s heavier and much harder to work with when it comes to drilling holes and trimming. There’s also the risk of cracking/shattering from rapid temperature differences or simply dropping the thing. Metal, especially aluminum would be the best option in most respects, but then I’d lose out on the visual spectacle, which is most of the point of this toy. This was the area requiring the most experimentation to find something I was happy with.

After safety, my second-highest priority is not pissing off my neighbors. They’re nice people, and I’d like to stay on their good side. The double bottle, hourglass shape with a medium-sized exhaust hole produces an impressive bang that’s great for parties, but bad for friendly relations with the other people who live nearby. As much as it pains me to ramp things down, I was going to be firing this thing off over and over as I tested it, so the opening on the business end had to be enlarged enough to keep the noise to a tolerable level. It also has the convenient side-effect of lowering the pressure within the bottles, so there’s less chance of explosive failure.

A minimalist assembly (torch, tube, bottle) and some fiddling with different hole sizes showed that I the volume level I’m shooting for required me to hack the entire bottom off of the end bottle. In fact, the bigger the end bottle the better. Even with a 1 QT bottle as the combustion chamber, I ended up using a full gallon jug as the bell end just to get it to more of a fwump than a bang.

I tried some smaller bottles to see if scaling the entire thing down might help, but wrapping the vinyl tubing into too tight of a coil collapses it into a flat oval, which has a noticeable impact on the flame front. It really wants as cylindrical of a path as it can get in order to propagate smoothly, otherwise it’s more prone to go out before reaching the end of the gun.

In the end, I settled on a pair of half gallon mason jars with the lids glued together with JB weld. The glass is thick enough to withstand the pressures I’m dealing with, but it’s also rather heavy. Drilling holes in these things took a lot of patience, especially the 50mm exit hole. This does, of course, mean that I have to be careful of breaking the glass from impacts or too rapid a temperature change. It’s winter and MAPP gas burns hot.

My attempts to keep the tubing compact revealed that fire does not like to make 90 degree corners, especially early on in the tube length. Not only did the back pressure create serious problems with the flame propagation through the rest of the tubing, it also blew a lot of the unburned gas back out of the handle through the air mixture holes. Not a good result on many fronts. In the end, I had to compromise between some wayward tubing and a 45 degree copper connector.

The vinyl tubing is 10 feet of 3/4 inch and 10 feet of 5/8 inch diameter. I’ve seen a lot of other people struggling to get their flame to travel more than 10 feet or so, and the reduced diameter for the second half of the trip seemed to solve that issue pretty well. It also speeds up the flame, which gives a nice building tension as you hear it wind up faster and faster before the pop.

Drilling holes for all the zip ties reminded me how badly I need a drill press. I’ve been putting off the expense for years, and I regret it every time I need to put a halfway decent hole in something. Good tools are always worth the expense. The “line” of holes in that square tube is shameful.

Wrapping the tubing around the mason jars and tightening down the zip ties required about 4 more hands than I actually possess. I made up for the missing hands with a lot of swearing and an hour or so more time than it should have taken.

After all that, I produced this masterpiece… well, wholly adequate… OK, just good enough not to blow my face off:

The flow rate is a bit finicky. It takes around 7 seconds to fill properly, but the amount of air mixed in with the gas isn’t exactly the same after every ignition, so it’s difficult to get a consistent pop out of it. I got the best performance at about 15 seconds into the below video. Still, for a week of sporadic experimentation, it’s a fun toy. So long as it doesn’t explode in my face one day.

My v 1.0 is not terrible, but there are still some tweaks needed before I’d be happy enough to call it complete. It’s pretty nose-heavy. The center of gravity is somewhere right around where the valve is. The handle with the ignition switch is on a pretty thin tube which is far too delicate to take the full weight of the gun, and my other hand has to operate the valve, so there’s really no good place to support the weight of the thing. Right now, I’m balancing most of the weight on the back of my right hand and trying to steady it with my left. This is an awkward arrangement at best. Adding a stock to the frame that could be braced on my forearm or shoulder should solve all of those issues, so that will most likely be the first thing I work on next.